The invention relates to a resistor, in particular an SMD resistor, and a corresponding production method according to the invention.
FIG. 4 shows an exemplary embodiment of a conventional SMD (Surface Mounted Device) resistor 1, which is marketed by the applicant and which in a similar form is described, for example, in DE 43 39 551 CI. The known SMD resistor 1 comprises a planar metallic substrate 2, which may be composed of copper, for example. In the production process an electrically insulating adhesive layer 3 is applied to the upper side of the substrate 2, and then serves to bond a resistive film to the upper side of the substrate 2. The resistive film is then structured by an etching process, so that a meandering resistance path 4 is formed on the upper side of the substrate 2. The resistor 1 is then covered by a protective lacquer 5, which electrically insulates the resistance path 4. Before completion, a transverse incision 6 is then made in the substrate 2, which divides the substrate 2 into two separate support elements 2.1, 2.2, thereby preventing a direct flow of current between the two support elements 2.1, 2.2. The support elements 2.1, 2.2 therefore here form the electrical connection parts of the SMD resistor 1, which can be soldered onto solder pads 7, 8, as is indicated schematically by the arrows in the drawing.
A disadvantage to the known SMD resistor 1 is the intricate electrical connection of the underlying support elements 2.1, 2.2 to the resistive film bonded on top, which forms the resistance path 4. For this purpose a conductive surface must first be achieved in preparation for a current-carrying, electroplated contact on the outer edge of the adhesive layer 3 (chemical through-hole plating), before then in a multistage electroplating process applying a layer of copper, which will reliably conduct the total current. This contact, however, is part of the current path through the SMD resistor and therefore also has an influence on the resistance of the SMD resistor 1, which in the case of low impedances with a resistance of less than 25 mΩ means that the resistance has to be adjusted on the separated individual SMD resistor 1, a resistance adjustment on a blank with multiple resistors in this case being precluded.
A further disadvantage of the known SMD resistor 1 stems from the incision 6 in the substrate 2, since the incision 6, for mechanical stabilizing of the SMD resistor 1, is filled with a lacquer or epoxy resin, which expands during the soldering-on process and leads to bending of the SMD resistor 1, the bending being virtually frozen in once the solder has solidified, and at very least leaving a visible defect in the finished component. This problem occurs particularly with the use of lead-free solders, which require a higher soldering temperature. In addition, a certain volume of lacquer is needed in the incision 6, in order to mechanically stabilize the SMD resistor 1 despite the presence of the incision 6, which in turn implies that the substrate 2 is relatively thick. In practice, the substrate 2 must therefore have a thickness of at least 0.5 mm, which places limits on the miniaturization of the SMD resistor 1. Regardless of the thickness of the substrate 2, the mechanical load-bearing capacity of the SMD resistor 1 is limited by virtue of the mechanical weakening introduced by the incision 6.
A further disadvantage of the SMD resistor 1 results from the high electroplating costs, which account for approximately 25% of the total production costs. These high electroplating costs stem from the fact that the lateral contact of the two support elements 2.1, 2.2 to the resistance path 4 must carry the full current flow, so that the demands placed on the density and the effective cross-section of the electroplated copper layer are relatively high. In addition, at low-impedance resistance values the influence of the copper on the electrical characteristics is not entirely negligible.
Finally the support elements 2.1, 2.2 as connection parts do not conform to the usual standard dimensions of solder pads, but are substantially greater in length. Any shortening of the two support elements 2.1, 2.2 and hence a widening of the incision 6, however, would lead to a further mechanical and thermal weakening and is therefore not possible.
FIG. 5 shows another type of a known SMD resistor 9, which is marketed by the applicant, a similar type also being described in EP 0 929 083 B1. The SMD resistor 9 comprises a planar, thin aluminum substrate 10, the substrate 10 in this type having no incision and hence no mechanical weakening. Bonded to the underside of the planar substrate 10 by an adhesive layer 11 is a resistive film 12, which is structured by an etching process and forms a meandering resistance path. Lamellar copper contacts 13 are applied to the underside on the narrow end sides of the SMD resistor 9, and form electrical contacts with lamellar connection parts 14, 15. Finally, the SMD resistor 9 of this type has a protective lacquer coating 16, 17 on the upper side and on the underside.
Of advantage in this type of the SMD resistor 9 is firstly the fact that the substrate 10 has no mechanical weakening, so that the ensuing problems described above are avoided.
A disadvantage of the SMD resistor 9, however, is the fact that the connection parts 14, 15 and hence also the soldering points are situated on the underside of the SMD resistor 9, where the soldering points are not open to visual inspection. Attaching soldering points laterally is not possible in the case of the SMD resistor 9, however, since the soldering points would otherwise create an unwanted electrical shunt via the electrically conductive substrate 10.
A further disadvantage of the SMD resistor 9 is that the substrate 10 of anodized aluminum is relatively hard, which means that when separating the SMD resistor 9 by sawing, the life of the saw blade is reduced. In addition, sawing off the individual SMD resistors 9 from an aluminum blank leads to an unwanted saw burr on the sawn-off SMD resistor 9, owing to the low melting point of the aluminum compared to copper.
Finally, applying the protective lacquer 6 to the upper side of the SMD resistor 9 and the inscription of the SMD resistor 9 leads to material-based production problems.
Another conventional type of SMD resistor finally comprises a planar ceramic substrate, which on its upper side carries a structured resistive film, the resistive film likewise forming a meandering resistance path. The electrical contact of the SMD resistor is here achieved by solder caps of a highly conductive, generally electroplate-reinforced, solderable metallic layer (for example nickel-chromium alloy), the solder caps being of U-shaped cross-section and enclosing the opposing narrow edges of the SMD resistor with a cap shape. The solder caps are here laterally accessible, so that when soldering up laterally visible soldering points are produced, which facilitate visual inspection of the soldered connections.
A disadvantage with this type, however, is the fact that the substrate is composed of ceramic and therefore has a relatively low thermal conductivity compared to copper (cf. FIG. 4) or aluminum (cf. FIG. 5) and a low coefficient of thermal expansion poorly suited to a normal circuit board. In addition, the resistive film is here located on the upper side of the substrate, which has the detrimental influences on the overall resistance previously described.
Similar resistors having a non-metallic support element are disclosed in US 2004/0252009 A1 and DE 30 27 122 A1, for example.
Finally, DE 196 46 441 A1 discloses a resistor, in which the connection parts, however, are attached solely to the underside, so that no visual inspection of the soldered connection is possible.
Proceeding from the known SMD resistor 9 according to FIG. 5, the object of the invention, therefore, is to eliminate the disadvantages of the SMD resistor 9, by facilitating visual inspection of the soldering points.
This object is achieved by a resistor and a production method according to the invention.